The field of art to which this invention pertains is adhesive compositions, particularly those compositions based on solid polymers in admixture with titanium chelate catalysts.
Pressure-sensitive adhesives, which are normally tacky materials adherent upon application of light pressure, are desirable in a number of industrial applications because they can be used with various materials and are easily adapted to production line techniques. In order to function satisfactorily as a pressure-sensitive adhesive, a composition must provide a combination of properties including good tack, adhesion and cohesive strength, and must adhere instantaneously when applied to substrates of differing surface characteristics. To be useful in many industrial applications, the bond obtained by the use of the adhesive must have sufficient strength to remain firm over relatively long periods and under different and sometimes severe environmental conditions.
Most pressure-sensitive adhesives, however, have low relative strength as compared, for instance, to thermosetting resin adhesives, because the desired tack and adhesive properties are ordinarily inconsistent with the high level of cohesive strength obtained with other types of thermosetting resin adhesive polymers. Because of this, attempts to provide pressure-sensitive adhesives having improved cohesive strengths by modifying the resin composition have not proved to be satisfactory with continuing problems being the tendency of the adhesive to lose strength upon aging and the unsatisfactorily low tack and adhesion which tend to be found with higher strength adhesives of this type.
Pressure-sensitive adhesives having improved cohesive strength properties are described in U.S. Pat. Nos. 3,769,254, 3,886,126 and 4,005,247. The improved strength properties are obtained by using, as the adhesive polymer, an acrylic polymer containing active hydrogen moieties and crosslinking the polymer by reacting with metal alkoxides. Preferred metal alkoxides are the titanate esters, and particularly the chelate esters of orthotitanic acid. However, it has been found that when the pressure-sensitive adhesive compositions which contain chelate esters of orthotitanic acid are placed in contact with unprotected iron or steel surfacess, highly colored iron complexes can form which render the adhesive unsuitable for many commercial applications. The adhesive also can cause corrosion of the metal surface and, in many instances, gelation of the adhesive polymer occurs at the metal surface.